Variable gain amplifiers (VGAs), for maintaining a desired level of output signals by adjusting gain, are widely used in radio frequency (RF) communication systems, in particular in transceivers receiving a signal which experiences rapid and wide variations in signal power.
In receivers such as those that may be used in various portable devices or base stations, it is often necessary to control the power of the demodulated signal for proper signal processing. Additionally, in transmitters such as those that may be used in various portable devices or base stations, it is also often useful to control the transmit power in order to avoid excessive interference from other equipment.
Reception and transmission power and gain control are typically performed by an automatic gain control (AGC) circuit using a VGA. It is generally desirable for the AGC to have high linearity and low noise, over a wide range of power levels, such that signals can be received and transmitted with little or no distortion. To achieve the desired AGC characteristics, the dB gain of the VGA should preferably be linearly changed according to a gain control signal over a wide dynamic range.
However, a VGA's performance may degrade significantly over a high dynamic range. For example, sensitivity of a VGA to low level signals may be reduced when the VGA is operating with a very high gain; on the other hand, the input signal may get lost in noise if there is insufficient gain for the VGA to amplify the input signal. Conventional VGA circuits have been found to exhibit deterioration in linearity at upper or lower ranges of gain.
Accordingly, solutions for achieving low noise, high linearity, and sufficient gain range for VGAs are desirable to provide adjustable gain in a variety of applications.